The SLS AMG and Ferrari 458 are both aimed toward a similar demographic and share some notable similarities. Both are powered by naturally aspirated V8's sending their power to the rear wheels through dual clutch transmissions. The Ferrari has a mid-engine layout however versus the front engine Mercedes and this being a Spyder it is a little heavier than the 458 coupe (about 110 pounds). Even with the additional weight the 458 Spyder still undercuts the SLS AMG by about 250 pounds

The cars are very evenly matched in the video despite the Ferrari being lighter. On paper, the motors are rated at similar output with the 6.2 liter V8 in the SLS rated at 563 horsepower and the 4.5 liter direct injected unit in the Ferrari rated at 562 horsepower.

So how is the SLS able to keep up despite more weight? The M159 V8 under the hood from all dyno results posted thus far is putting out more rear wheel horsepower than the 458 in stock form. With more power and torque it is able to negate the weight difference.

Very good runs showing evenly matched cars, come to your own conclusion on the actual power output of the two cars.

If you’ve still got any doubt that the Mercedes SLS AMG is a supercar, if not an exotic, then take a look at this video of Mercedes-AMG’s hellion matched up against a Ferrari 458. On paper, the engines of the two cars are pretty much even, both being highly-tuned V8s, and being a couple of horsepower shy of 565 stallions. However, the stats will show that the Ferrari is about 250 pounds lighter than the SLS AMG. That’s the equivalent of one or two passengers.
Which makes it a bit of a surprise why the Ferrari doesn’t get a big jump on the SLS on the acceleration runs. Torque and gearing would play a part. As would the supposedly better drivetrain efficiency of the SLS, which enables it to put out more rear-wheel horsepower to the driven wheels. So is Ferrari overstating the 458′s output? Or is AMG being coy about what its engine really makes?

Sound familiar to anyone? If you are going to rip me off at least cite me as a source, don't make it blatant.

And it isn't supposedly better drivetrain efficiency for the SLS it's that the M159 simply puts more power to the wheels than the Ferrari 458. Maybe if you looked at the dyno numbers on your own like I did you would get it right...

Newer 458's dyno at ~480 whp. 2010 models had transmission issues and the one that Fabspeed tested was a 2010 too. It only managed to do 450 whp. Even with 480 whp, it seems low for a the claimed 562 hp from Ferrari. These cars are over-rated.

Another point to be made is the weight difference. a Spyder weighs ~3,130 lbs dry. Add 240 lbs of fluids to it and you get a 3,370 lbs car (per Ferrari manual). With a full tank of gas, it can go up to 3,510 lbs (It holds 23 gallons of gas). Daytona interior? That's another 35 lbs. So you will get a 3,545 lbs car with a full tank.

What a SLS weighs? 3,650 lbs, right? I looked up and it seems these cars put down ~10-15 more hp to the wheels and i think it's enough to cover the 105 lbs weight difference.